1. Field of the Invention
This invention relates to eyepiece lenses for use in telescopes, binoculars, microscopes and, more particularly, to wide-angle eyepiece lenses which cover a field of about 60.degree..
2. Description of the Related Art
The eyepiece lens is used to enlarge the apparent field of view when an object on a plane is observed. Of its types, the Erfle and orthoscopic ones are widely known.
For example, Japanese Laid-Open Patent Application No. Sho 49-115558 proposes an eyepiece lens of the orthoscopic type comprised of two groups with four members.
Japanese Patent Publication No. Sho 63-7363 discloses an eyepiece lens of the Erfle type comprising three groups with five members, wherein an increase of the eye-relief is achieved.
Anther Japanese Laid-Open Patent Application No. Sho 60-159719 has made a proposal for widening the apparent angular field to 60.degree. or so, while reducing the astigmatism at the edge of the field to be covered. Another Japanese Patent Publication No. Sho 63-10409 shows an eyepiece lens of widened apparent field of about 60.degree. with relatively small distortion over the entire area of the field to be covered.
Such an eyepiece lens in the Japanese Laid-Open Patent Application No. Sho 60-159719 suffers very large distortion of 16% at a semifield of 30 degrees. Though it is acceptable for astronomical telescopes, the use of this eyepiece lens in the ordinary binocular gives a problem of distorting the image largely.
Also in the Japanese Patent Publication No. Sho 63-10409, the eyepiece lens, though having as small distortion as 3% at a semifield of 30 degrees, gives rise to a problem of increasing the curvature of field and the astigmatism greatly.
In general, with the use of the above-described forms of either the Erfle or the orthoscopic type in the eyepiece lens, when widening the apparent field angle up to 60.degree. and elongating the distance from the back of the eyepiece lens to the pupil of the observer's eye (hereinafter, this distance is called the "eye-relief"), while still maintaining the entire system to be constructed in compact form, the number of lens surfaces of convergent refractivity for the off-axial principal ray is caused to increase. Therefore, it becomes very difficult to correct well all aberrations to obtain a high optical performance.
Meanwhile, an eyepiece lens whose angular field is about 60.degree. with the number of constituent lenses reduced to four or five is proposed in Japanese Laid-Open Patent Application No. Sho 55-24084.
Another proposal for improving the imaging performance of the eyepiece lens with the limitation of the lens number to 4 is disclosed in Japanese Laid-Open Patent Application No. Sho 60-48729.
However, the eyepiece lens proposed in the Japanese Laid-Open Patent Application No. Sho 55-24084, though being of wide-angle, has large field curvature and large astigmatism. In actual use, therefore, the image would probably be blurred at the edge of the field.
The eyepiece lens proposed in the Japanese Laid-Open Patent Application No. Sho 60-48729, though being improved in imagery, has a narrow field angle of 50.degree.. In the embodiment using four constituent lenses, on the other hand, there is a problem that the eye-relief is as short as 60% of the focal length.